Many oil reservoirs have been discovered which contain vast quantities of oil, but little or no oil has been recovered from many of them because the oil present in the reservoir is so viscous that it is essentially immobile at reservoir conditions, and little or no petroleum flow will occur into a well drilled into the formation even if a natural or artificially induced pressure differential exists between the formation and the well. Some form of supplemental oil recovery process must be applied to these formations which decreases the viscosity of the oil sufficiently that it will flow or can be dispersed through the formation to a production well and therethrough to the surface of the earth. Thermal recovery techniques are quite suitable for viscous oil formations, and steam flooding is the most successful thermal oil recovery technique yet employed commercially.
Steam may be utilized for thermal stimulation for viscous oil production by means of a steam drive or steam throughput process, in which steam is injected into the formation on a more or less continuous basis by means of an injection well and oil is recovered from the formation from a spaced-apart production well. The injected steam not only serves to drive the oil into the production well but it also condenses giving up its heat to the formation thereby reducing the viscosity of the oil and enhancing its recovery. Injection of steam and production of oil is continued until steam breakthrough occurs at the production well. Continued injection of steam into the formation after steam breakthrough will accomplish very little economical oil recovery because of the unfavorable ratio of oil to water at the production well.
The present invention is a modified steam injection process that reduces the steam injection period and the amount of steam injected by introducing nitrogen into the formation after steam has been injected for a predetermined period of time. The replacing of steam by nitrogen decreases the cost of production of each barrel of oil because the cost of generating nitrogen used for flooding the formation in the present invention is only one-third of the price for steam generation. Also, the use of nitrogen can also prevent formation damage, well corrosion, and the government restriction of air pollution in such areas as California.